The petroleum industry continues to expand deepwater exploration and drilling efforts in many areas of the world. As the drilling water depth increases, the potential for natural gas hydrate formation during drilling operations also increases.
Gas hydrates are solid, ice-like crystals that form under elevated pressures and at moderately low temperatures. Gas hydrates consist of water molecules which form five (pentagon) and six (hexagon) membered polygonal structures which combine to form a closed structure (often called a "cage"). These "cages" totally enclose or trap a gas molecule. At high pressures, multiple "cages" tend to combine to form larger cages enclosing gas molecules. The resulting large crystalline assemblies are thermodynamically favored at elevated pressures. Under sufficient pressure, gas hydrates will form at temperatures well above the freezing point of water.
Primary promoters of gas hydrates are gas with "free" water present at or below its water dew point, low temperatures, and high pressures. Secondary promoters are high velocities, pressure pulsations, any type of agitation, and the introduction of a small crystal of a hydrate. During deepwater drilling operations, all of the primary gas hydrate promoters are present. The drilling fluid supplies the "free" water; low temperatures are encountered on the sea floor [32-45.degree. F. (0-7.2.degree. C.)]; and, the hydrostatic head of the fluid produces high pressures. The formation of gas hydrates may become a problem in a pressure and temperature range between about 500 psia at 35.degree. F. (1.67.degree. C.) or lower to about 8000 psia at 80.degree. F. (26.67.degree. C.) or lower, particularly between about 1000 psia at 35.degree. F. (1.67.degree. C.) or lower to about 6000 psia at 80.degree. F. (26.67.degree. C.) or lower.
The formation of gas hydrates in deepwater drilling operations creates safety concerns and can prevent proper operation of the blowout preventer during well-control operations, causing costly delays in controlling the wells. The use of a 20% sodium chloride/PHPA drilling mud system suppresses gas hydrate formation by 24-28.degree. F. (13.3-15.6.degree. C.) below that of freshwater. Often, however, it is necessary to suppress the temperature of hydrate formation even further--for example, during disconnects. "Spotting" of certain materials reportedly is effective to temporarily remove hydrates from various equipment and/or lines; however, spotting often is not performed until after gas hydrates have formed and caused the very delays sought to be avoided. Drilling fluids are needed which inherently provide the necessary hydrate suppression without the need for spotting and which still have desirable rheology and fluid loss control properties for deepwater environments.